Problem with GetItem in production

I'm a newbie and I'm developing a simple program. Probably my question is silly, but I'm wasting time looking for a solution...

At this very moment the program can do just some easy stuff. It can perform a simple search on a eBay site (using findItemsByKeywords or similar functions) and retrieve the result. The I want to show also other details of a single item, as the description, and therefore I use GetItem.

Now, when I use those functions in the sandbox, everything works fine. But when I try to use them in Production, findItemsByKeywords works fine, but GetItem returns an error code 127 (invalid application name).

The question is: in order to use GetItem (Trading API) in Production should I register my application? And, if yes, where do I can register the application (link, please)?